A beginner/intermediate VM, with only a few twists by author g0tmi1k. Apparently 2 ways to get limited shell and 3 ways to get root.
Author KookSec created this machine to help others learn some basic CTF strategies and some tools. This machine is aimed to be very similar in difficulty to those found in the OSCP – which it was. This is a large post, but lists the 3 privilege escalation paths. What a great challenge.
The Bandit wargames by OverTheWire is aimed at beginners and is lots of fun. It teaches the basics and many useful commands. Finding the solution is one thing, however eliminating other solutions and what you learn on the way is a great experience. Its highly recommend you try to solve these yourself before looking at the solutions. Note no keys or spoilers are found here.